1. Field of the Invention
The present invention relates generally to a method and apparatus for coupling spacecraft, and more particularly to a method and apparatus which combines capture and berthing techniques to effect coupling between spacecraft.
2. Description of the Related Art
Spacecraft coupling apparatus provide a practical way for mechanically joining spacecraft for the logistical support and transfer of crew members between two or more spacecraft, for the mutual support of two or more spacecraft by a sharing of their individual facilities and capabilities, and for the on-orbit repair or assembly and structural expansion of spacecraft. Other on-orbit operations that require spacecraft to be joined include the addition or exchange of modules containing expendable supplies and equipment needed for space-borne experiments and manufacturing processes, the collection of products and byproducts of space-borne experiments and manufacturing processes, and the on-orbit repair and maintenance of spacecraft.
Many of these on-orbit operations can optimally be implemented by crew members moving between the spacecraft through a passage extending between the coupling apparatus of the joined spacecraft. The use of properly designed coupling apparatus for creating a pressurized, shirt-sleeve environment in the passageway between the joined spacecraft is particularly important to the safety of the crew, as the joining of such coupling apparatus can preclude the need for potentially hazardous extravehicular excursions performed by crew members. Furthermore, by eliminating the need for wearing cumbersome space suits when traveling between spacecraft, crew members will be less restricted in their work environment and, therefore, better able to perform a variety of tasks, many of which require the use of a considerable degree of manual dexterity that would otherwise be hindered by the wearing of a cumbersome space suit.
In the past, spacecraft have generally been coupled to one another in one of two ways: docking and berthing. In both processes, the interfacing surface of coupling apparatus carried by respective spacecraft must be fully coupled, or superimposed, by a force supplied to the spacecraft. The processes of docking and berthing differ from one another principally in the manner in which the coupling force is applied to the spacecraft to be joined thereby. In general, a docking process between two spacecraft is initiated by purposefully colliding the spacecraft with one another so as to obtain the necessary coupling force from the relative momentum imparted to both spacecraft by propulsion means such as thrusters on one or both of the spacecraft. In contrast to docking, berthing provides a coupling procedure in which one spacecraft is joined to another by the force and directional control applied by an external positioning system, such as a remotely controllable manipulator arm carried by one of the two spacecraft to be joined.
The use of an external positioning system, such as a remotely controllable manipulator arm, during a berthing process affords a considerable, inherent advantage over docking by virtue of the ability to control various aspects of relative vehicle movement throughout the phases of spacecraft closure and contact. As it is not always possible to position the coupling apparatus of the respective spacecraft to be joined between the centers of gravity of the two spacecraft, the force resulting from the relative motion between the spacecraft incident to a docking procedure can be sufficient to create an undesirable moment about a center axis of either spacecraft and, therefore, cause either spacecraft to "jackknife" and lose axial alignment. The development of such moments can be avoided during berthing due to the relatively lesser forces developed incident to berthing procedures.
Docking procedures can be further disadvantageous for the relative momentum required for superimposing the interfacing surfaces of the respective spacecraft docking apparatus can result in the transmittal of a considerable load to the structures of both spacecraft that support the respective interfacing surfaces. Further, the physical loads imparted to docking spacecraft by relative motion can result in aberrant spacecraft motion. Thus, spacecraft which undergo docking are generally equipped with devices for dampening motion and for structurally reinforcing the spacecraft. Further, the interfacing surfaces themselves must be designed so as to withstand the forces created by the relative momentum of the docking spacecraft. For all of the foregoing reasons, the configuration of a spacecraft to accommodate coupling by way of docking results in an appreciably greater degree of overall complexity and weight as opposed to spacecraft that are configured to accommodate coupling by way of berthing.
While the structures of berthing spacecraft are exposed to generally lighter physical loads than those of docking spacecraft, the external positioning system used with berthing spacecraft can present disadvantages because of its complexity. External positioning equipment such as robotic arms are generally remotely controlled and must be designed with a sufficient range of motion so as to permit the equipment to attain proper alignment in order to grasp the target object. However, the design, development and implementation of a device which has sufficient mobility and tactility maneuver to grasp a target is both time consuming and expensive.